In addition, I should mention that in Windows C:\ is not the system root. Many computers have only one hard drive and often there is only one partition - definitely the case with old computers. Then users should be able to create their own data folders. I would be very annoyed if I were restricted to the standard "My Documents" folder for doing this. In fact, I never put any of my own data there at all. But since creating a folder C:\junk does not in any way interfere with anything, I see no reason why this should not be allowed.
However, if you are managing a computer with multiple users and want to restrict their privileges, then follow logicearth's advice and create standard accounts for them.